In recent years, an organic electroluminescence (EL) display device using OLEDs (organic light emitting diodes) which are self light-emitting elements has received attention as a flat display device, and thus has been developed actively. Since the OLEDs are self light-emitting elements, the organic EL display device has features such as no backlight required, a wide viewing angle, and fast response which is desirable for motion pictures, compared with a liquid crystal display device that controls the light intensity from a backlight by liquid crystal cells.
As with liquid crystal display devices, organic EL display devices can adopt, as their drive method, a simple (passive) matrix method and an active matrix method. Although a display device of the simple matrix method is simple in structure, the display device has a problem of, for example, having difficulty in implementing a large-area and high-resolution display device. Hence, in recent years, active development has been performed on display devices of the active matrix method that control a current flowing through a light-emitting element by an active element, e.g., a thin-film transistor (TFT), provided in the same pixel circuit as the circuit where the light-emitting element is provided.
In a display device of the active matrix method, each pixel is composed of an OLED serving as a display element; and a pixel circuit that supplies a drive current to the display element. An organic EL display device performs a display operation by controlling the luminance of the display elements. A pixel circuit includes, for example, a driving transistor connected in series with a display element; a switching transistor connected between a data line and a gate of the driving transistor; a capacitor connected between the gate and source of the driving transistor and holding a voltage according to a video signal; and the like.
Meanwhile, unlike a liquid crystal cell, an OLED is a current-driven type self light-emitting element. Thus, during a display operation, the driving transistor connected in series with the display element needs to be in an ON state all the time to allow a current to continuously flow. Therefore, with the passage of time, characteristic degradation of the driving transistors, such as a decrease in mobility and an increase in threshold voltage, is observed.
Such characteristic degradation of the transistors brings about a change in the amount of current during operation, which is recognized as variation in luminance or burn-in in each pixel of the organic EL display device. Particularly, transistors formed of amorphous silicon have a very high degree of threshold voltage shift, which is a great hindrance to practical use.
In recent years, a method has been proposed to form, in a pixel, a threshold voltage detection circuit that detects, a threshold voltage shift of a transistor formed of amorphous silicon (see, for example, Non-Patent Document 1). In this technique, by providing, in each pixel, a threshold voltage detection transistor that can short-circuit between the gate and drain of a driving transistor, a gate voltage is discharged during a threshold voltage detection period provided separately from light emission time, so that a threshold voltage of the driving transistor can be stored as a potential at the other end of a holding capacitance.
In the above-described threshold voltage detection circuit, as a preparation operation to detect a threshold voltage, the operation of bringing the gate potential of the driving transistor to a sufficiently high potential (reset operation) is required. Normally, in the reset operation, by turning on the threshold voltage detection transistor to raise the potential of a power supply potential Vss, a current flows through the driving transistor in an opposite direction to that for light emission and the gate of the driving transistor is charged through the threshold voltage detection transistor.
In such a pixel structure, however, the ease of current flow through the driving transistor itself affects the charge of the gate potential of the driving transistor. Therefore, if a threshold voltage shift of the driving transistor occurs and thus the current supply capability of the driving transistor decreases, the potential at which the gate is reset also decreases, causing an error in the accuracy of compensation for a threshold voltage shift of the driving transistor.